Nonribosomal peptide synthetases (NRPSs) produce a wide array of small and structurally complex peptides that have therapeutic potential. The system enables the incorporation of nonproteinogenic amino acids into the polypeptide. Polyketide synthetases (PKSs) are a family of enzymes or enzyme complexes that produce polyketides. Integration of PKSs into the NRPSs system further increases the variety of polypeptides that can be produced by these systems. Recent studies are aimed at exploiting NRPSs for producing peptide libraries that can be screened for therapeutic applications.1-9 
Unlike linear peptides, cyclic peptides are restrained to fewer conformations that facilitate their interaction with their molecular target.10-18 These structural constraints provide resistance to proteases, extreme pH, and temperature.10, 19 These attributes make them one of the most promising scaffolds for pharmacophores. Synthetic design of cyclic peptides is hindered by regioselectivity.
Classical total synthesis of peptides by solid phase or solution phase peptide synthesis followed by subsequent cyclization reactions requires the addition and removal of protecting groups at the right stages to drive the cyclization among the correct residues.8 Even with these considerations, proper cyclization is hindered by intermolecular interactions and entropically disfavoured pre-cyclization conformations resulting in a vast mixture of compounds or low yields. Microorganisms ensure the formation of a functional cyclic peptide conformation by enzymatically catalyzing the cyclization and release of the peptide with regioselectivity using a cyclase thioesterase.1, 7 The cyclase thioesterase is often located at the C-terminal end of the last NRPS involved in the synthesis of the peptide and is referred to as the TE (Thioesterase) domain.
The TE domain can hydrolyze the bound peptide as a linear peptide or it can catalyze an intramolecular reaction resulting in the formation of a cyclic peptide. At present, very little is known about the cyclization mechanism of peptides. The crystal structure of the surfactin peptide cyclase provided the first basic understanding of its mechanism of action.20, 21 The peptidyl chain bound to 4-phosphopantetheine cofactor (ppan) that is attached to the thiolation (T)-domain is transferred to a serine in the adjacent TE domain. Ser80 is part of a catalytic triad of residues (His 207 and Asp107) in the surfactin cyclase. His207 and Asp107 activate the Ser80, facilitating the transfer of the peptidyl chain to the TE domain. Once the peptide is transferred to the TE domain, the cyclase binding pocket enables proper orientation and cyclization of the peptide substrate. The enzyme was found to share structural homology to α,β-hydrolase family. The lack of water in the binding cleft of the cyclase, which prevents hydrolysis, is the significant alteration from the hydrolase family that gives the cyclase thioesterase its ability to form cyclic peptides.
Occidiofungin is a broad spectrum nonribosomally synthesized cyclic antifungal peptide that has submicro/nanomolar activity and low toxicity.19, 22-26 An interesting feature in occidiofungin's biosynthetic pathway is the presence of two putative thioesterases. One is present as an independently expressed thioesterase, OcfN, and the other is a C-terminal TE domain of OcfD. There remains a need for the production of anti-fungal agents that have increased cidal activity against various fungi.